The invention relates to a color filter determining apparatus, wherein a measurement of the basic color components occurs by means of at least one photoelectric transducer, which is a part of a measuring circuit.
For the determination of the filter values required for the copying of color transparencies, such as negatives, filter setting apparatus have been successful whose measuring principle resides in the measuring of reciprocal proportions of the basic color components. In most of the known apparatus of this type, photoelectric transducers, usually photoconductive cells, are employed as the measuring receptor, which forms a branch of a measuring bridge. For the determination of the filter values necessary for the copying of a negative, the light penetrating through the negative to be copied is conducted through the color filter to the measuring receptor and the measuring bridge is balanced by potentiometers, one color filter and one potentiometer pertaining to each basic color to be measured. Scales are provided for the control elements and are calibrated in filter values. In this manner it is possible to determine for the copying procedure the filter values necessary for the correction of possible color errors in the negative from the positon of a potentiometer pertaining to a certain color component, indicated after the measurement.
Calibration of known color filter determining apparatus takes place by the balance of the measuring bridge for each of the basic colors, a negative being employed for the measuring which has a color distribution characteristic for a certain number of negatives to be processed. The balance state of the measuring bridge achieved in this manner determines the reference values for all negatives of a certain series to be measured thereafter.
This reference value is determined either by individual registering potentiometers in the measuring bridge or by scales with adjustable zero points for the balancing potentiometers.